$\left(9x + 3\right)\left(-9x - 7\right) = \ ?$
Explanation: $= 9x \cdot \left(-9x - 7\right) + 3 \cdot \left(-9x - 7\right)$ $= \left( 9x \cdot -9x \right) + \left( 9x \cdot -7 \right) + \left( 3 \cdot -9x \right) + \left( 3 \cdot -7 \right)$ $= -81x^2 + \left( 9x \cdot -7 \right) + \left( 3 \cdot -9x \right) + \left( 3 \cdot -7 \right)$ $= -81x^2 + \left( -63x - 27x \right) + \left( 3 \cdot -7 \right)$ $= -81x^2 - 90x + \left( 3 \cdot -7 \right)$ $= -81x^2 - 90x - 21$